Individual origin VS Median point
I think he has made a small mistake. Neither of these two settings will give the desired result, on the left Median Point (will place the center of the scale at the center of all the selections), on the right Individual Origins (each selection will have its own center of scale):

Actually as you can see when he scales, he constrains the scale on the Z axis (X axis and Y axis are visible, not Z axis), that's why it works as expected.